Abstract: In Qinghai,the loess is widely distributed and coal reserves are rich. Rapid consumption of coal resources leads to increased production of fly ash. Therefore,many scholars have studied fly ash as a cementitious material and added it to cement soil. The shear strength of fly ash cement soil is poor. At the same time,due to the problems of loess collapsibility,the strength of fly ash cement soil can rapidly decrease. This paper aims at the influence of different curing ages on the shear strength of fly ash cement soil. It carries out unconsolidated and undrained triaxial shear tests to analyze the effect of curing age on fly ash cement soil from the perspective of macro mechanics. It combines with SEM test and XRD test to analyze the internal structure and material composition of the specimen from a microscopic perspective. Experimental results show that: from a macro perspective,the overall stress-strain curve of fly ash cement soil shows strain softening. The shear strength of the sample gradually increases with the increase of curing age and the shear strength is the largest at the 28 days. Simultaneously,due to the increase of the curing age,the mutual chemical reactions between the substances in the sample can continue. The longer the curing age of the sample,the stronger the cementation between the substances inside the sample,which can cause the internal friction angle and cohesive force of the sample to gradually increase. From a micro perspective,with the extension of curing time,the crystalline material (Ettringite) and gelling material (C-S-H gel) generated inside the sample can fill the large pores inside the sample. These materials can adhere to each other,and cause the sample to become denser and the shear strength increases. This paper provides a test basis for the shear strength of solidified loess with fly ash and other materials,offers reference for the application of industrial by-products such as fly ash in engineering,and has reference significance for the utilization of fly ash and environmental protection.